How do I solve two simultaneous equations?

Supposing we have two equations x+2y=5 and 3x-y=10 and we want to find x and y we can use the following techinque. Multiply both the RHS and the LHS of the second equation by 2, giving us 6x-2y=20. Then add this to the first equation, giving us 7x=25 thus x=25/7. The substitute the value for x to find y: 25/7 +2y=5 => y=5/7. Alternatively, we can manipulate the first equation to give us x=5-2y and substitute in the second equation: 15-6y-y=10 => -7y=-5 =>y=5/7 and then substitute in the first equation to find x. Remember, to find the value of n variables we need n equations, except in special cases.